It is known to use blank metal sheets to manufacture various types of metal components. Such blank metal sheets are bent and formed into the desired shape. The bending of a metal sheet comprises plastically deforming the metal and changing its shape. The metal is stressed beyond the yield strength, but below the ultimate tensile strength so as to not break the metal sheet into pieces. For example, in tube making, blank metal sheets are bent so as to form hollow beams. As another example, metal sheets are bent to form various automotive parts, such as tie bars in the radiator support assembly.
In prior art, the bending or folding of metal sheets is accomplished by the use of press brakes. A typical prior art press brake comprises: (i) a moveable upper tool called a punch or ram; and (ii) a stationary lower tool called a die. The die has a V-shaped opening on its upper surface. A metal sheet is placed flat on the upper surface of the die over the V-shaped opening. The punch is moved downwards along a vertical axis to push down against the metal sheet and presses the metal sheet into the V-shaped opening. As a result of this downward pressure from the punch, the metal sheet is bent into a desired shape, a V-shaped bend.
Several problems arise when attempting to bend blank metal sheets by using a press brake. First, when the moveable punch presses the metal sheet down into the V-shaped opening, there is significant rubbing and sliding between the surface of the metal sheet and both the surfaces of the moveable punch and the stationary die, and this often results in shoulder marks or scratches on the surface of the metal sheet. Therefore, refinishing is required after the bending procedure.
Alternatively, the surface damage caused by press brake tooling can be reduced using urethane tooling or polymer die inserts. However, such provisions increase the cost of manufacturing.
Further, as the moveable punch is released from the bent metal sheet, there is a tendency for the metal sheet to lose some of its bent shape. This problem is known in the art as “springback”. The amount of springback depends on several factors including the type of material, thickness, grain and temper. Springback is known to be very pronounced when the metal sheet comprises high-strength steel and aluminum alloy. Generally, the springback ranges from about 5 to 10 degrees.
In an effort to reduce the problem of springback, press brakes are often designed to bend the metal sheet to a greater angle than desired, thus taking the springback into account. However, this requires the carrying out of complex calculations to predict the degree of springback. Further, because there are so many factors involved in determining the amount of springback, including material, thickness, grain and temper, it is very difficult to accurately predict. Moreover, the costs and labour time required to make the necessary adjustments to the press brake, including the size and shape of the punch, the size and shape of the V-shaped opening in the die, and the downward pressure of the punch, are high.
For many metal components, it is desirable to form the metal component from a variable gauge metal sheet, or a metal sheet having a varying thickness profile. Such variable gauge metal sheets have the advantage of being able to form components which are lighter in weight because the sheet is thinner in areas which will be subjected to a lesser load than the load applied to thicker areas of the sheet. Also, less metal material is required to form the metal component. By selectively varying the thickness profile of the metal sheet, the function of the component can be optimized. Further, the varied thickness profile of the metal sheet offers greater possibilities for form design.
However, the bending of variable gauge metal sheets runs into the same problems as the bending of regular metal sheets which are mentioned above. Further, the bending of variable gauge metal sheets is even more difficult because it is more complicated to bend such metal sheets along a straight fold line where the thickness of the metal sheet varies along said fold line.
As mentioned above, the amount of springback varies according to the thickness of the metal sheet being bent. Therefore, where a metal sheet has a variable thickness profile, and the metal sheet is bent along a fold line where the thickness of the metal sheet varies along said fold line, the amount of springback will also vary along the fold line. This results in the metal sheet having a warped shape, with a greater amount of springback on one side of the metal sheet than the other. Further, it is difficult to design a press brake which takes into account springback when the amount of springback varies along the fold line. In this case, the amount of springback must be predicted at every point along the fold line.